Question

Quinones formed during enzymatic browning react with _______.

• A. sugars
• B. lipids
• C. amino acids.
• D. minerals

Hint:

Enzymatic browning involves both enzymatic steps (monophenolase and diphenolase activities of PPO to make quinones) and non-enzymatic steps (reaction of quinones with amino acids to make brown melanins).

Answer 1

The Correct Option is C

Step 1: Understanding the Question:
The question asks about the secondary chemical reaction that occurs after the enzymatic formation of quinones during the browning of bruised fruits and vegetables.

Step 2: Key Concepts and Approach:
Enzymatic browning is catalyzed by polyphenol oxidase (PPO).
PPO oxidizes natural phenolic compounds into highly reactive ortho-quinones.
These quinones are unstable and undergo non-enzymatic polymerization reactions to form dark brown pigments called melanins.

Step 3: Detailed Explanation:

• Primary Reaction:
Polyphenols + $O_2$ \xrightarrow{PPO} Quinones + $H_2O$.

• Secondary Non-Enzymatic Reactions:
Quinones are electrophilic and react with nucleophiles present in the cell.
The primary nucleophiles in foods are the amino groups ($-NH_2$) of amino acids, peptides, and proteins.
This reaction forms substituted catechol-amino acid complexes, which then undergo further oxidation and polymerization.

• Melanin Formation: These complex networks form the final dark brown polymers (melanins) that appear on the surface of sliced apples, bananas, and potatoes.

• Comparison with other components: Sugars are involved in Maillard browning (non-enzymatic), but they do not readily react with quinones under these conditions.

Step 4: Final Answer:
Therefore, quinones formed during enzymatic browning react with amino acids.